The present invention relates to an arrangement for the support and guiding means of a wire electrode used for effecting a cut by electro-erosion in a workpiece.
In electro-erosion apparatus provided with a wire electrode, the electrical discharges occur between an electrode workpiece and the wire electrode fed into the machining zone between a pair of electrode guiding and support means, the workpiece and the wire electrode being displaced relatively to each other about two axes, for example, by means of numerically controlled servo motors. The inputs to the servo motors are obtained from an appropriate program providing a pre-established machining path.
The accuracy of machining of a wire electrode electro-erosion apparatus is greatly dependent upon the electrode wire support and guiding means. It is particularly difficult to feed a wire electrode beyond its support and guiding means in a perfectly rectilinear path. Errors in the precision with which the wire electrode is supported and guided are due to many causes of a mechanical nature, and more particularly are due to variations in the load applied to the wire when the wire holders are subjected to a change in trajectory, and to variations in friction between the wire and the surfaces in engagement with the wire, with the result that unequal frictions exerted by the guiding surfaces on the wire create a torque causing a lateral displacment of the wire being fed beyond the guiding surfaces. Diameter variations of the wire while being fed through the workpiece, combined with the roughness or the wear of the wire surface in the machining zone may also cause errors between the actual positioning of the wire at a given moment and the desired positioning according to its programmed path.
It is known to support and guide a wire electrode by holding its peripheral surface engaged with a pair of reference flat surfaces disposed at a predetermined angle relative one to the other. However, such an arrangement does not permit to completely eliminate positioning errors, especially in structures in which the element urging the wire in engagement with the reference surfaces in addition acts as a contact for supplying electrical current to the wire electrode. The coefficient of friction of this element, made of a current conductive metal, is different from the coefficient of friction of the reference surfaces of the wire electrode guides which are generally made of sapphire.
In the event of variations in the diameter of the wire, such an arrangement causes an error in the position of the active surface of the electrode, and such error is substantially larger along the bisector plane of the two wire guide surfaces than in a plane perpendicular to either surface. If, for example, the guide reference surfaces are disposed at a relative angle of 60.degree., the error in position of the wire in the bisector plane is three times what is would be in a plane perpendicular to either surface. It is therefore readily apparent that the diverse errors affecting the precision of guiding of the wire electrode, whether considered singly or in combination, are produced in a direction which is dependent upon the geometry of the wire electrode guiding means. Consequently, the importance of the errors affecting the machined surface of the workpiece is a function of the orientation of the electrode holder and guide relative to the machining path.
Heretofore, the prior art system for controlling the relative position of a workpiece and of a wire electrode do not take into consideration the orientation of the wire holder and guide relative to the machining path, and the reference surfaces guiding the wire are indifferently positioned relative to the machining path. The disadvantages and inconvenience of the prior art arrangements are remedied by the present invention whose principal object is to provide means for greatly improving the machining accuracy as a result of providing a wire electrode holder and guide arranged such that the most important positioning errors of the wire take place in a predetermined direction relative to the machining path, such that they have no influence upon the quality and accuracy of the machined surface.